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that were immersed in acetate buffer were 10 times stiffer than those that
were bathed in neutral or alkaline buffers. In the second study,
37
a Young's
modulus was determined to be 0.4 MPa.
The mechanical properties of secretory vesicles have likewise been
studied in the AFM. The molecular mechanism that underlies the release of
secretory products from cellular vesicles is still incompletely understood,
although the stiffness of these intracellular organelles is believed to play an
important role in the process. In 1997, Laney
measured the Young's
modulus of cholinergic synaptic vesicles that had been isolated from the
marine ray. The value lay between 200 kPa and 1.3 MPa, depending on the
composition of the bathing medium.
Lipid rafts are small cholesterol-enriched membrane domains, which have
been implicated in several physiological and pathological processes. Until
recently, their properties were unknown. To ascertain whether the stiffness
et al.
38
Figure 16.7.
AFM image of an axon showing
topography, elasticity and positions of
raft domains. The stiffness of the membrane is colour-coded (blue: soft; red: hard).
The arrows point to lipid rafts. Scan size: 2
M
m (kindly provided by Dr. C. Roduit).
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